For golfers, it is an eternal theme to extend flight distance of a ball and shoot the ball in an aimed direction and angle. Therefore, it is important to use a golf club suited for one's own swing.
Selecting a golf club suited for a golfer is generally referred to as fitting. In order to effectively perform this fitting, it is necessary to take into consideration various factors such as the total weight of a golf club, the weight of a club head, and the length of the golf club. In particular, the success or failure of fitting is greatly influenced by physical properties of a shaft of a golf club.
Until now, for fitting a golf club shaft, many techniques focusing mainly on flexural rigidity of a shaft have been proposed. Yet, techniques focusing on twist rigidity (torque) of a shaft have not been much proposed.
However, during a golf swing, although it is true that the swing leads to bending of a shaft, twist motion is generated at the same time of the bending of the shaft since the center of gravity of a head is not in line with the shaft axis and since the wrist of a golfer moves during the swing. This twist motion greatly also relates to flight distance of a hit ball, directionality, and ease of swinging a golf club. In particular, directionality and ease of swinging are greatly influenced by the twist rigidity of the shaft.
Regarding the torque of the shaft, in conventional techniques, the torque has been determined empirically from a head speed upon impact and an apparent swing tempo. However, such techniques often have to depend on the experience and intuition of the person (fitter) who performs the fitting, and have a problem of the selection result being not objective and varying from person to person.
As a response thereto, a technology has been proposed in which a user is asked to swing a golf club having a sensor mounted thereto, and a golf club is designed or selected using obtained sensor data (e.g., refer to. Patent Literature 1).
In a golf club designing device/program set forth in Patent Literature 1, flex and twist rigidity of a shaft is established using the sensor data. From the sensor data, a swing response curve obtained by expressing the skill of the user as a function is calculated, and a motion of a golf club grip obtained from the swing response curve is provided as displacement data to calculate a motion of a club head.